The present invention relates to radar signal processing. More particularly, the present invention relates to the multistatic adaptive pulse compression processing of radar signals.
The increased desire for ever greater sensor coverage inevitably results in proximate radars overlapping ( at least partially) their respective operating frequency bands and as such are sources of mutual interference. This problem is exacerbated by the allocation of portions of the frequency spectrum previously allocated for radar to wireless communications. It is therefore desirable to provide concurrent, shared-spectrum radar operation in order to mitigate mutual interference, as well as to exploit the potential benefits that such an arrangement would provide, including aspect angle diversity, greater area coverage with shorter revisit times, and anti-stealth sensing capability.
It is well known that two or more radars operating in relatively close proximity, at the same time, and in the same spectrum will interfere with one another—often to the point of achieving complete RF fratricide. This is because it is impossible to generate waveforms that are orthogonal to one another at all possible respective delays and Doppler frequency shifts. The result is that a relatively large target return associated with one of the received signals can mask target returns from the other received signals.
A significant amount of work has been done to design sets of waveforms/matched filter pairs that possess suitable ambiguity and cross-ambiguity characteristics. Representative approaches are described in “A study of auto- and cross-ambiguity surface performance for discretely coded waveforms”, L. O Carroll, D. H. Davies, C. J. Smyth, J. H. Dripps, and P. M. Grant, IEE Proc. F; Commun., Radar, and Signal Process., 137, No. 5, pp. 362-370, October 1990, and “Multi-parameter local optimization for the design of superior matched filter polyphase pulse compression codes”, C. J. Nunn and L. R. Welch, IEEE Intl. Radar Conf, pp. 435-440, (2000). The subject waveforms are designed such that the overall ambiguity ( i.e. range sidelobe levels and cross-correlations) is minimized on average over all delay/Doppler shifts and cross-correlations. However, as long as the radar receivers rely on standard deterministic pulse compression techniques (matched filtering or Least-Squares based mismatched filtering), there remains the distinct possibility that a small target will be masked by large targets that may exist in nearby range cells (within the same range profile) or by large targets in another range profile from which the reflected signal arrives nearly coincident in time at the receiver and whose corresponding waveform possesses a non-negligible cross-correlation with the waveform associated with the small target of interest. The combination of range sidelobes and waveform cross-correlation can collectively be considered as multistatic interference.
Conceptually, in order to mitigate the masking problem, a receive filter for a particular waveform at a particular range cell must be closely matched to the given transmitted waveform-while also cancelling the interference from targets in nearby range cells (range sidelobes) as well as from target returns from other received signals (waveform cross-correlations). Hence, the receive filters must be adaptive to the actual received signals since the appropriate receive filter will be unique for each individual range cell associated with each received signal.
An approach for the monostatic radar case known as Reiterative Minimum Mean-Square Error (RMMSE) estimation, described in U.S. Pat. No. 6,940,450, issued Sep. 6, 2005 and incorporated herein by reference, is capable of accurately estimating the range profile illuminated by a radar by suppressing range sidelobes to the level of the noise floor. This is accomplished by adaptively estimating the appropriate receiver pulse compression filter to use for each individual range cell. Furthermore, the RMMSE algorithm, which has also been denoted as Adaptive Pulse Compression (APC) when applied to the radar pulse compression problem, has been shown to be robust to rather severe Doppler mismatch.
It would be desirable to provide an adaptive radar processing system that can resolve a radar target in the presence of multiple radar return signals occupying a shared frequency spectrum.